XVII 



ANATOMY OF OPHIOTHR1X 



479 



jaw. 



One of the commonest British Ophiuroids is Ophiothrix 

 fragilis (Figs. 207, 208), which is found in swarms in shallow 

 water off the west coast of England and Scotland. We may 

 therefore select it as the type, and, since the arm is the most 

 characteristic organ of an Ophiuroid, we may commence by study- 

 ing it. Speaking generally, an Ophiuroid either drags itself 

 forward by two arms and pushes itself by the other three 

 (Fig. 207), 1 or else it drags itself by one and pushes with the other 

 four (Fig. 217). The arms during this process are bent into 

 characteristic curves, by the 



straightening of which in the ., v P 



posterior arms the animal is 

 pushed onwards, whilst the 

 intensification of these curves 

 in the anterior arms causes 

 the animal to be dragged 

 forwards. The grip of the 

 arm on the substratum is 

 chiefly in the distal portion P ^ 

 of the curve. The alteration 

 of the curvature is due to the 

 contraction of the muscles on 

 one side of the arms. There 

 is no ambulacral groove such 

 as is found on the under side 

 of the arms of all Asteroidea, 



for the arm is completely ensheathed by four series of plates, an 

 upper row of dorsal plates, an under row of ventral plates, 

 and two lateral rows of lateral plates. The last named, which 

 in all probability correspond to the adambulacral plates of 

 Starfish, bear each a transverse row of seven spines with 

 roughened surfaces ; these enable the animal to get a grip on 

 the substratum over which it moves. The podia in Ophiuroidea 

 are termed " tentacles " ; they are totally devoid of suckers, 

 being simple conical papillae used as sense-organs, and are of 

 little, if any, service in locomotion. They issue from openings 

 called " tentacle -pores " situated between the edges of the 



Fig. 208 



Oral view of the disc of Ophiothrix 



fragilis. g.b, Opening of the genital bursa ; 



m.p, madreporite ; pod, podia ; t.p, tooth - 



papillae ; vp. ventral plates of the arms. 



x 1. 



1 This figure does not show the animal's attitude during forward progression 

 quite correctly. The tips of the two anterior arms should be bent outwards, not 

 inwards as in the figure. 



